This invention relates to a brake actuator intended for use primarily, although not exclusively, with an internal shoe drum brake of the leading/trailing type, usually known as a simplex brake.
One characteristic of drum brakes is that they operate with a degree of self-energization or servo derived from the rotating brake drum and this tends to increase the braking force on the leading shoe and reduce the braking force on the trailing shoe, as is well understood. A result of this is that the friction material of the leading shoe wears away at a faster rate than that of the trailing shoe with consequent inconvenience to users in terms of servicing requirements.
A proposal to deal with the unequal wear problem is illustrated in British Patent No. 456238. This provides the trailing shoe with an automatic adjuster device coupled by a flexible drive element to the leading shoe so that adjustment is applied in equal amounts to both shoes. Although this partially corrects the unequal wear, the aforesaid characteristic of unequal braking forces at the respective shoes, which gives rise to the unequal wear, remains unchanged.
Another partial solution to the problem has been attempted by using friction linings of different thickness on the leading and trailing shoes, but this again deals with the effect of the problem rather than its fundamental cause and results in the inconvenience of "handed" shoes.
In a further prior proposal, a hydraulic actuator is provided with opposed pistons having different working areas, the pistons being arranged so that the larger actuating force is applied to the trailing shoe. Whilst this can in theory balance the forces applied respectively to the leading and trailing shoes, it is not entirely satisfactory because the geometry of the brake varies during use and it is difficult in practice to match the forces to the theoretical ideal.